This invention represents an improvement over U.S. patent application Ser. No. 890,490 filed in the U.S. Patent and Trademark Office on Mar. 27, 1978 and owned by the assignee of the present invention.
Tubing heretofore used in the conveyance of fluids and/or fluid transmission signals under pressure has in the main been of the type wherein the wall of the tube is made singularly from either a polymeric or a metallic material and the outer diameter of the tube has been sized such that, dependent upon whether the tube is made singularly from a metallic or polymeric material, it can be attached to conventional fittings generally peculiar to each. To a lesser extent, tubes having an extremely thin inner metallic layer formed by longitudinally folding metallic tapes into a tubular shape and extruding a polymeric material thereabout have also been used for such purposes. Tubes having a longitudinally folded metallic inner layer can be found, for example, in U.S. Pat. Nos. 3,233,036 or 3,806,358 and are also presently made and sold by the assignee of the present invention under the trade name "Dekabon". Tubes having an inner layer formed by the folding of metallic tapes have the disadvantage of having an inner exposed seam which can lessen their thermal resistance and mechanical handling capabilities and which may also provide a point of attack in applications requiring the conveyance of corrosive fluids. The mechanical handling capabilities of tubes having an inner layer formed by the longitudinal folding of a metallic tape are necessarily limited, particularly in smaller tube sizes, due to the fact that metal tapes are generally required to be relatively thin in order to fold them longitudinally into a tubular shape at attractive production rates. Generally, tubular shapes formed by the longitudinal folding of metallic tapes are made in such a manner as to provide an overlap between their oppositely facing longitudinally extending edges for the purpose of mechanically interlocking or otherwise adhesively affixing the folded tape into a closed tubular configuration. As a result of this practice, the radial thickness of the folded metal tape is necessarily larger in the region of the overlap than elsewhere about the circumference of the tube. Nonuniformity in radial thickness of metallic tubular shaped forms can, and often does lead to greater tendency for fluid leakage under pressure in the region of fitting attachment. Tubes made in accordance with the present invention do not possess the aforesaid disadvantages in that they comprise a tubular shaped seamless, as welded, or welded and drawn inner layer of metallic material having a substantially uniform radial wall thickness around its circumference; an outer tubular shaped layer of polymeric material of substantially uniform radial wall thickness around its circumference disposed in encompassing relationship about the inner layer and of a magnitude such that its radial wall thickness is 50% or more of the total radial thickness of the wall of the tube; a polyamide based hot melt adhesive interposed between the inner and outer layers which bonds them together and which preferably contains an amount by weight of piperazine used in the process of making the polyamide to promote its bonding characteristics without adversely affecting its water resistance; and an outer tube diameter sized during the process of making the tube such that the tube can be attached, without further dimensional alteration, to fittings standardly used in the industry. The term "seamless, as welded, or welded and drawn", as herein used, shall not mean metallic tubing which has been formed by the longitudinal folding or spiral wrapping of metallic tapes having abutting or overlapping edges which are mechanically interlocked or adhesively adhered together so as to result in a discontinuous or interrupted wall in the form of a seam and which is further characterized as not having a substantially uniform radial wall thickness around its circumference. The term "seamless, as welded, or welded and drawn", as herein used, means tubing that has been formed by extrusion, drawing, or welding and drawing, and the like, such that the tube wall is integrally sealed and of substantially uniform radial wall thickness around its circumference.
The coating of metallic tubes and pipes with polymeric materials has long been recognized as a method of providing protection from corrosive environments. U.S. Pat. No. 2,447,420 disclosed, for example, the coating of rigid electrical conduit with a polymeric material and U.S. Pat. No. 2,724,660 disclosed an immersion process of applying a protective jacketing to flexible metal tubes while in a shaped configuration. In U.S. Pat. No. 3,815,640, a synthetic resin coated pipe is disclosed that has annular projections at regular intervals for use as a supporting pole in agricultural applications and U.S. Pat. No. 3,502,492 discloses the method of electrostatically coating metal substrates with epoxy resin and polyvinyl chloride powder. Such art, however, does not teach or suggest tubing of the type made in accordance with the present invention in that it is devoted to the coating, with polymeric materials, of standardly recognized metal tube wall thicknesses resulting in oversized tube outer diameter in relationship to what is normally a fixed standardized inner diameter, or vice versa, and generally requires dimensional alteration such as, for example, skiving away of the polymeric material in order to make fluid-tight seals when attached to conventional fittings.
Tubes made in accordance with the present invention provide a means of transferring fluid and/or fluid transmission signals under pressure from one location to another with the corrosion resistance and integrity of seamless, as welded, or welded and drawn metallic inner liner and the protective benefit of an outer polymeric coating bonded thereto while maintaining a standardized outer diameter such that the tube can be attached, without further dimensional alteration, to fittings standardly used to couple equivalently dimensioned tubes having a singular polymeric or metallic wall.